The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, increase of oil price is triggering a hard competition in enhancing fuel consumption of a vehicle.
In this sense, research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages.
In order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts.
In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages has been developed.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
In addition, a recent eight-speed automatic transmission typically shows a gear ratio span in a level of 6.5 to 7.5.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.